Process of making paper pulp



013in 3l, 19,33. R H, MCKEE 1593125904 PROCESS OF' MAKING PAPER PULP Filed Jan. 4, 1953 QAAAAAAAAA Patented Oct. 31, 1933 UNITED STATES PATENT OFFICE 16 Claims.

This invention relates to a process of making paper pulp, and more particularly to a pr cess of preparing pulp suitable for use in the nufacture of paper from vegetable bres suli as wheat, rice, oat and ax straws, bagasse, corn and cotton stalks, and the like, i. e. agricultural by-products.

The principal object of this invention is to provide a commercially practicable process of preparing paper pulp from fibrous vegetable materials of the above referred to character.

An important object of the invention is to improve the present practice of the so-called alkali-chlorine gas process of making paper pulp.

A further object of the invention is to provide a process of making paper pulp from fibrous vegetable materials by the use of alkali and chlorine gas wherein the use of high pressure and high vacuum treatments are avoided.

A further object of the invention is to provide a process of making paperpulp from brous vegetable materials involving the use of alkali and chlorine gas wherein the period of the treatment with alkali is materially shorter than in other alkali-chlorine gas processes heretofore employed.

A further object of the invention is to provide a continuous alkali-chlorine gas process of preparing pulp from vegetable fibres.

Other objects and advantages of the invention will become apparent during the course of the following description.

As is well known, the major proportion of paper pulp produced is prepared from wood. However, in view of the steadily decreasing supply of suitablepulp wood, and because of the availability and cheapness off/fibrous agricultural by-products such as straws, corn stalks andthe like, much work has been done in recent years in an eiort to develop commercial processes of producing satisfactory paper pulp from fibrous vegetable materials. Perhaps the greatest success along this line has been obtained with various modifications of the so-called alkali-chlorine gas process which, broadly, involves initially digesting the fibrous material with an alkali, subjecting the alkali-treated material to the action of chlorine gas, and washing the final product with a weak alkali solution. i`

This type of process was originally derived from Ithe classic Cross and Bevan method of analysis of vegetable products for cellulose. The first practical embodiments of this general process were devised in Italy by Cataldi and in France by deVains. The Cataldi process, particularlyas modified by Pomilio, has met with a substantial degree of commercial success, particularly in those localities where vegetable fibres are readily accessible at a low cost and pulp wood is either not readily obtainable or is too expensive to permitl of its use in the manufacture of pulp on an economical basis.

However, the alkali-chlorine gas processes heretofore employed possess several practical disadvantages which have prevented this type of process from coming into wider use. For example, in the Cataldi process the-straw or other raw material is initially subjected to a relatively long alkali digesting treatment under high pressure. Thereafter the product is treated with chlorine gas and because of the diiculty with temperature control, the chlorinev gas treatment is carried on for only a short period when the material is treated with water, the chlorine and water treatments being repeated several times. Moreover, both the chlorine and water treatments in the Cataldi process involve the use of high vacuum. Finally, the lCataldi process is not a continuous process but is practised on batch scale and involves an unreasonably long period of time for completing the process.

.As a result of extensive research directed to overcoming the disadvantages of the prior alkali-chlorine gas processes of making pulp from vegetable fibres, I have developed an improved process which is continuous throughout, which permits exact temperature control, which materially decreases the time of the material in process, which is capable of operation in cheaper and more simple apparatus than that used in prior processes ofthe character referred to, and which in all other respects is adapted for best commercial practice.

In its broad aspects, my process consists of cooking straw or other vegetable br'e in a dilute alkali for a relatively short period at atmospheric pressure, subjecting the alkali-treated stock to the action of a mixture of chlorine gas and air at atmospheric pressure while spraying the stock with water, and washing the resulting product with a weak alkali solution, the stock being moved in a continuous stream from the beginning to the4 end of the process.

In the specific practice of the process, which is hereinafter described in conjunction with the form of apparatus shown diagrammatically in the accompanying drawing, wheat straw or other vegetable fibre of the type set forth above is preliminarly cut to 2 or 3 inches in length, dusted,

and Washed. While the straw may be cut into pieces as short as 1 inch or as long as 5 inches, I have found it advantageous to use straw of 2 or 3 inches in length. After its preliminary treatment the straw is carried on the conveyer 10 and delivered into an alkali cooking tank 11 where it is treated with a very weak alkali, say a 1 to 2 per cent. caustic soda solution, for about an hour or a little longer, depending upon the raw material under treatment, at a temperature slightly below the boiling point of water. The mass in the tank 11 is preferably heated to the desired temperature by means of steam introduced at a point adjacent the bottom of the tank from the steam pipe 12.

The alkali-treated material is continuously removed from the bottom of the tank 11 on a conveyer 13 and during its passage on the Vconveyer is freed from excess alkali by means of press rolls 14 after which it is washed by water from sprays 15, the waste wash liquor being carried tothe sewer through the waste pipe 16. The washed material is delivered from the end of the conveyer to press rolls 17 where the excess liquid is pressed out, after which the material is passed into a tower 18 which is preferably built of concrete and lined with acid-resistant tile. This tower may be of the general character of. a conventional bleaching tower used for bleaching wood pulp. The tower 18 is connected at its lower end with a similar tower 19, a screw conveyer 20 being arranged to pass the material from the bottom ofthe tower-18 up through the tower 19 Where a screw conveyer 21 delivers the material into a tower 22 which is similar to the towers 18 and 19.

The tower 22 is connected at its lower end with a tower 23, and a conveyer 24, similar-to the conveyer 20, is arranged to take the stock from the lower end of the tower 22 and pass it into and up through the tower 23. 'Above the towers are arranged water sprays '25, 26, 27 and 28. Below the conveyers 20 and 24 are arranged perforated tile bottoms 29 and 30 below which are basins 31 and 32 discharging into pipes 33 and 34 which are adapted to deliver liquid from the basins to a sewer. Injectors 35 are arranged in the walls of the towers as shown in the drawing. While thermometers, valves and other conventional accessories are employed these are not indicated in the drawing.

As will be apparent from the foregoing, the alkalme-treated stock passes down the tower 18, up the tower v19, ydown the tower 22, and up the tower 23. If the speed of transfer is slow enough,

only the first two towers may -be needed, but .f

as the processv is ordinarily operated four towers are preferably employed. As the stock passes in its tortuous course through the several towers, which ordinarily requires from two to four hours, there is injected into the stock through the injectors 35 a mixture of chlorine gas and air. In the preferred practice I employ a mixture of one volume of chlorine gas to two to four volumes of air. I have discovered that the use of air with the chlorine gas has the advantage of modifying the treatment and preventing it from becoming localized. Moreover, an almost equally important result of using air with the chlorine gas is that it serves to carry awa'y the heat of reaction of the chlorinefon the lignin. I n addition, the presence of the oxygen of the air with the chlorine appears to produce an increased effect. This is somewhat analogous to the increased eifect noticed when chloride of lime SO- lutions are used for removing color and lignin in the bleaching of paper pulp and air is blown into the solution under treatment.

During the treatment of the material in the towers with the mixture of chlorine gas and air, water is sprayed into the top of the towers from the sprays 25, 26, 27 and 28. and runs down through the stock. This water serves a double purpose in the process. In the first place, it serves to dissolve the lignin chloride, i. e. the yellow-colored compound formed by the action of chlorine on the non-cellulosic materials in the raw material, the resulting solution passing out of the towers through the perforated bottoms 29 and 30 to the sewer through the pipes 33 and 34. In the second place, the water serves to provide exact temperature control during the treatment of the stock with the mixture of chlorine gas and air. For example, by increasing or decreasing the amount of water used as a spray in the treating towers, the operation mayV be conducted at exactly 60 C., as in preferred practice. Hence, the continuity of the chlorine treatment is not interrupted as in similar processes heretofore employed where the heat of reaction resulting from the action of the chlorine on the lignin of the brous material required the chlorine treatment to be periodically discontinued to permit cooling of the mass.

The amount of chlorine introduced into the stock in the treating towers may be varied according to the amount of caustic employed in the initial treatment. The amount of chlorine varies inversely with the amount of caustic us'ed'. For example, if the straw or other material is initially treated with a 2 per cent.' caustic solution instead of a 1 per cent. or a'1/2 p er cent. solution, then the amountpf chlckine -required will be decreased. Even withfa 1 percent. caustic solution, if -the fibrous material is cooked for l hour, more chlorine will be required than if the fibrous material is cooked for 2 hours in the preliminary cook. Accordingly, the plant can, if desired, sell from its electrolyticplant making caustic soda and chlorine from common salt, either caustic soda or chlorine and use the excess of the unsold material to accomplish the pulp cooking. The quality of the pulp seems to be the same whether the amount of chlorine used is large or small, the caustic soda, of course, at the same time being small or large, respectively.

In some instances it may be found desirable to press the stock during its passage from the tower 19 to the tower 22, this having the advantage of pressing out the spent treating liquid @and speeding up the treatment in the tower 22.

Moreover, if the fibrous material employed, such as straw, is unusually coarse it may be found Ydesirable to subject the fibrous material during its passage from the towe` 19 to the tower 22 to a rubbing action whereby fresh surfaces of the bre will be available for nal treatment. An edge runner, ball mill or other well known form of rubbing apparatus may be used for this purpose.

As stated above, the towers must be provided with acid-resistant linings.' Moreover, the conveyers 20, 21 and 24 must be resistant to the hydrochloric acid solution and chlorine with which they come in contact. I have found that screw conveyers of the ordinary type but coated with rubber or well plated with silver are not expensive and serve admirably for the purpose as the silver is not materially affected by the chemical agents present as long as it is wet.

The crude pulp discharged from the tower 23 is passed between press rollers 36 to press out the liquid present. The resulting material is then discharged into a tank 37 where it is given a weak alkali wash as in similar processes heretofore employed. The pulp is continuously removed from the bottom of the tank 37 by means of a conveyer 38. During its passage on the conveyer 38 the pulp is passed between press rolls 39 to remove the excess liquid present and the resulting product is sprayed with water from the sprays 40,.-.tl'ie waste wash liquor being discharged to the sewer through the pipe 4l. The pulp discharged from the upper end of the conveyer 38 may be treated in any desired manner, the pulp ordinarily being bleached, washed, and dried in any conventional manner.

While I have described in detail the preferred practice 'of my process and a convenient form of apparatus for use in connection therewith, it is to be understood that the details of procedure of the process and the-form of apparatus employed may be variously modified without departing from the spirit of the invention or the scope of the subjoined claims.

I claim:

l. In a process of the character described, the step which comprises subjecting alkali-treated fibrous Vegetable'material to the action of a miX- ture of chlorine gas and air.

2. .In a process of the character described, the

step which comprises subjecting alkali-treated fibrous vegetable material in moist condition to the action of a mixture of chlorine gas and air in the proportions of one volume of chlorine gas to two to four volumes of air.

3. In a process of the character described, the step which comprises subjecting brous vegetable material which has been cooked in a weak solution of caustic soda to the action of a mixture of chlorine gas and air in the proportions of one volume of chlorine gas to two to four volumes of air.

4. In a process of the character described, the step which comprises subjecting alkali-treated fibrous vegetable material to the action of a mixture of chlorine gas and air while passing a current of water in contact therewith.

5. In a process of the character described, the step which comprises subjecting alkali-treated brous vegetable material to the action of a mixtu'e of chlorine gas and air, in the proportions of one volume of chlorine gas to two to four v olumes of air, while passing a current of wate tl'r'rough the mass under treatment.

- 6. The process of making pulp from fibrous' vegetable material which comprises cooking the material in a weak alkali solution, subjecting the alkali-treated material to the action of a mixture of chlorine gas and air, and washing the l product Y The process of making. pulp from fibrous vegetable material which comprises cooking the material in a weak solution of caustic, subjecting I the resulting product to the action of a mixture of chlorine gas and air, and removing water soluble impurities from the resulting product.

8; The process of making pulp from fibrous vegetable material which' comprises cooking the material in a weak alakali-solution, subjecting tne alkali-treated material to the action of a mixture of chlorine gas and air while passing a current of water through the mass being treated, and treating the resulting product with a weak alkali solution.

9. The process of making pulp fibrous vegetable material which comprises cooking the material in a dilute solution of caustic soda, subjecting the cooked product to the action of a mixture of chlorine gas and air in -the proportions of one .volume of chlorine gas to two to four volumes of air, contacting the resulting product with Water to remove water soluble impurities therefrom, and treating the resulting product with a weak alkali solution.

10. In a process of the character described wherein alkali-treated fibrous vegetable material is subjected to the action of chlorine gas, the improvement which comprises simultaneously removing water soluble material from the product during the treatment with the chlorine gas and controlling the temperature of the reactionl mass by passing a current of water in regulated amount through the mass.

11. The process of making pulp from fibrous vegetable material which comprises cooking the material in a weak alkali solution, subjecting the alkali-treated product to the action of a mixture of chlorine gas and air, and simultaneously removing Water soluble material from the product and controlling the temperature of the reaction mass by passing in contact therewith a current of water in regulated amount.

l2. The process of making pulp from fibrous vegetable material which comprises cooking the material in a dilute solution of caustic, subjecting the thus treated product to the action of a mixture of chlorine gas and air in the proportions of one volume of chlorine gas to two to four volumes of air, and simultaneously removing water soluble material from the product and controlling the temperature of the reaction mass by passing in contact therewith a current of water in regulated amount.

13. The process of making pulp from fibrous vegetable material which comprises cooking the material in a dilute solution of caustic soda, subjecting the thus treated product to the action of a mixture of chlorine gas and air in the proportions of one volume of chlorine gas to two to four volumes of air, removing water soluble material from the product and controlling the temperature of the reaction mass by passing in contact therewith a current of water in regulated amount, and treating the resulting product with a weakxalkali solution.

'14. The process of making pulp from brous vegetable material which comprises passing a current of fibrous Vegetable materialfthrough an alkali cooking bath, passing a current of the alkalitreated product through an atmosphere 'containing a major proportion of air and a minor proportion of chlorine gas, and contacting the moving current of material with a current of water.

15. The process of making pulp from fibrous vegetable material Whichcomprises passing a current of brous vegetable material through a cooking bath of a hot dilute caustic soda solution, passing a current of the material out of said bath and through an atmosphere of chlorine gas and air in the proportions of one volume of chlorine gas to two to four volumes of air, contacting the current of material under treatment with water during the 'said treatment with chlorine gas and air, and passing a. current of the thus treated product through a weak alkali solution:

16. The process of making pulp from brous vegetable material which comprises preliminarily chlorine gas to two to four volumes of air, simultaneously removing water soluble material from the product and controlling the temperature of the reaction mass by passing therethrough water in regulated amount, removing the excess liquid from the thus treated material, treating the resulting product vwith a Weak alkali solution, and washing the resulting pulp. t

RALPH H. McKEE. 

